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><H1
CLASS="SECT1"
><A
NAME="TUTORIAL-INHERITANCE"
>3.6. Inheritance</A
></H1
><P
>    Inheritance is a concept from object-oriented databases.  It opens
    up interesting new possibilities of database design.
   </P
><P
>    Let's create two tables:  A table <CODE
CLASS="CLASSNAME"
>cities</CODE
>
    and a table <CODE
CLASS="CLASSNAME"
>capitals</CODE
>.  Naturally, capitals
    are also cities, so you want some way to show the capitals
    implicitly when you list all cities.  If you're really clever you
    might invent some scheme like this:

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE capitals (
  name       text,
  population real,
  altitude   int,    -- (in ft)
  state      char(2)
);

CREATE TABLE non_capitals (
  name       text,
  population real,
  altitude   int     -- (in ft)
);

CREATE VIEW cities AS
  SELECT name, population, altitude FROM capitals
    UNION
  SELECT name, population, altitude FROM non_capitals;</PRE
><P>

    This works OK as far as querying goes, but it gets ugly when you
    need to update several rows, for one thing.
   </P
><P
>    A better solution is this:

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE cities (
  name       text,
  population real,
  altitude   int     -- (in ft)
);

CREATE TABLE capitals (
  state      char(2)
) INHERITS (cities);</PRE
><P>
   </P
><P
>    In this case, a row of <CODE
CLASS="CLASSNAME"
>capitals</CODE
>
    <I
CLASS="FIRSTTERM"
>inherits</I
> all columns (<TT
CLASS="STRUCTFIELD"
>name</TT
>,
    <TT
CLASS="STRUCTFIELD"
>population</TT
>, and <TT
CLASS="STRUCTFIELD"
>altitude</TT
>) from its
    <I
CLASS="FIRSTTERM"
>parent</I
>, <CODE
CLASS="CLASSNAME"
>cities</CODE
>.  The
    type of the column <TT
CLASS="STRUCTFIELD"
>name</TT
> is
    <TT
CLASS="TYPE"
>text</TT
>, a native <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
    type for variable length character strings.  State capitals have
    an extra column, <TT
CLASS="STRUCTFIELD"
>state</TT
>, that shows their state.  In
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>, a table can inherit from
    zero or more other tables.
   </P
><P
>    For example, the  following  query finds the  names  of  all  cities,
    including  state capitals, that are located at an altitude
    over 500 feet:

</P><PRE
CLASS="PROGRAMLISTING"
>SELECT name, altitude
  FROM cities
  WHERE altitude &gt; 500;</PRE
><P>

    which returns:

</P><PRE
CLASS="SCREEN"
>   name    | altitude
-----------+----------
 Las Vegas |     2174
 Mariposa  |     1953
 Madison   |      845
(3 rows)</PRE
><P>
   </P
><P
>    On the other hand, the  following  query  finds
    all  the cities that are not state capitals and
    are situated at an altitude of 500 feet or higher:

</P><PRE
CLASS="PROGRAMLISTING"
>SELECT name, altitude
    FROM ONLY cities
    WHERE altitude &gt; 500;</PRE
><P>

</P><PRE
CLASS="SCREEN"
>   name    | altitude
-----------+----------
 Las Vegas |     2174
 Mariposa  |     1953
(2 rows)</PRE
><P>
   </P
><P
>    Here the <TT
CLASS="LITERAL"
>ONLY</TT
> before <TT
CLASS="LITERAL"
>cities</TT
>
    indicates that the query should be run over only the
    <CODE
CLASS="CLASSNAME"
>cities</CODE
> table, and not tables below
    <CODE
CLASS="CLASSNAME"
>cities</CODE
> in the inheritance hierarchy.  Many
    of the commands that we have already discussed &mdash;
    <TT
CLASS="COMMAND"
>SELECT</TT
>, <TT
CLASS="COMMAND"
>UPDATE</TT
>, and
    <TT
CLASS="COMMAND"
>DELETE</TT
> &mdash; support this <TT
CLASS="LITERAL"
>ONLY</TT
>
    notation.
   </P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>     Although inheritance is frequently useful, it has not been integrated
     with unique constraints or foreign keys, which limits its usefulness.
     See <A
HREF="ddl-inherit.html"
>Section 5.8</A
> for more detail.
    </P
></BLOCKQUOTE
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